Liquid crystal display and method for manufacturing same

ABSTRACT

A liquid crystal display is provided which is capable of reducing the occurrence of defective display due to variations in the initial alignment direction of a liquid crystal alignment control film in a liquid crystal display of an IPS scheme, realizing the stable liquid crystal alignment, providing excellent mass productivity, and having high image quality with a higher contrast ratio. The liquid crystal display has a liquid crystal layer disposed between a pair of substrates, at least one of the substrates being transparent, and an alignment control film formed between the liquid crystal layer and the substrate. At least one of the alignment control films  109  comprises photoreactive polyimide and/or polyamic acid provided with an alignment control ability by irradiation of substantially linearly polarized light.

This application is a Continuation application of application Ser. No.13/212,072, filed Aug. 17, 2011, now allowed, which is a Divisionalapplication of application Ser. No. 12/781,815, filed May 18, 2010, nowU.S. Pat. No. 8,025,939, which is a Continuation application ofapplication Ser. No. 10/537,825, filed Jun. 8, 2005, now U.S. Pat. No.7,718,234, the contents of each of which are incorporated herein byreference in their entirety. No. 10/537,825 is a National Stageapplication, filed under 35 USC 371, of International (PCT) ApplicationNo. PCT/JP03/15658, filed Dec. 8, 2003.

TECHNICAL FIELD

The present invention relates to a liquid crystal display of a so-calledIPS (In-Plane Switching) scheme in which an electric field substantiallyin parallel with a substrate is applied to a liquid crystal layer foroperation, and to a production process thereof.

BACKGROUND ART

In general, display of a liquid crystal display is realized by applyingan electric field to liquid crystal molecules in a liquid crystal layersandwiched between a pair of substrates to change the alignmentdirection of the liquid crystal molecules and utilizing the resultingchange in the optical property of the liquid crystal layer.Conventionally, a liquid crystal display of a so-called active drivetype having a switching device such as a thin-film transistor for eachpixel is represented by a TN (Twisted Nematic) display scheme in whichan electrode is provided for each of a pair of substrates sandwiching aliquid crystal layer between them, the direction of an electric fieldapplied to the liquid crystal layer is set to be substantiallyperpendicular to the interface between the substrates, and the opticalrotatory power of the liquid crystal molecules constituting the liquidcrystal layer is utilized to achieve display. In the liquid crystaldisplay of the TN scheme, a small viewing angle is regarded as thegreatest problem.

On the other hand, “Patent Document 1,” “Patent Document 2,” “PatentDocument 3,” “Patent Document 4,” “Patent Document 5” and the like havedisclosed an IPS scheme in which an inter-digital electrode formed onone of a pair of substrates is used to produce an electric field havinga component substantially in parallel with the substrate surface torotate liquid crystal molecules constituting a liquid crystal layer in aplane substantially in parallel with the substrate and the birefringenceof the liquid crystal layer is used to realize display. The IPS schemehas advantages such as a wider viewing angle and a lower load capacitydue to the in-plane switching of the liquid crystal molecules ascompared with the conventional TN scheme. The IPS scheme is consideredas a new and promising liquid crystal display which will replace the TNscheme and has made rapid advances in recent years. In addition, anothertype of the IPS scheme has been disclosed in “Patent Document 6” inwhich at least one of paired electrodes for applying an electric fieldto a liquid crystal layer is made of a transparent conductive film toimprove transmittance.

The liquid crystal display of the IPS scheme (abbreviated asIPS-TFT-LCD) with favorable viewing angle characteristics (luminancecontrast ratio, tone and color reversal) and bright display representsprospective technology for monitors or televisions with a large displayarea. In the liquid crystal display, an alignment control film providedwith a liquid crystal alignment control ability is formed on theinterface between a liquid crystal layer and each of a pair ofsubstrates sandwiching the liquid crystal layer between them. However,to put IPS-TFT-LCDs for supporting large screens of 20 inches or moreinto practice use in the future, it is necessary to develop a newstructure and process for large-size displays (large panels).

In particular, for an IPS-TFT-LCD having many stepped structures on asurface opposite to a liquid crystal layer, it is difficult to performuniform alignment processing on an alignment control film over a largescreen. A margin in performing the alignment processing on the alignmentcontrol film is significantly smaller than that of the conventional TNscheme, especially a normally open type TN scheme which is predominantat present (bright display at low voltage and dark display at highvoltage). The reasons for the small margin include three pointsdescribed below as (1) to (3).

(1) Stepped Structure

In the IPS-TFT-LCD, it is necessary to provide a number of elongatedelectrodes (which may be referred to as inter digital electrodes) havinga width of approximately several microns in principle. This causesminute stepped structures to be formed therein. The height of the stepdepends on the thickness of the electrodes or the shapes of variousfilms formed thereon, and typically is equal to 0.1 micron (mμ) orlarger. An alignment control film (also referred to as an alignmentfilm) made of a polymer film such as polyimide is formed in theuppermost layer of those films.

In conventional mass production technology, the alignment control filmis subjected to rubbing processing to provide a liquid crystal alignmentability (initial alignment). Meanwhile, a cloth for the rubbing isformed by binding thin fibers with a thickness of approximately 10 to 30microns. Essentially, each of the thin fibers provides shearing force ina predetermined direction for a local portion of the alignment film toperform the processing of giving the liquid crystal alignment ability.While very thin fibers of approximately several microns are present asthe fibers, such very thin fibers have not been put into practical usesince rigidity for providing certain frictional force is required forthe rubbing. The interval between the electrodes in the IPS scheme isapproximately 10 to 30 microns which is substantially the same as thediameter of the fibers, so that sufficient rubbing is not performed nearthe steps and misalignment tends to occur. The misalignment leads toreduced image quality such as a higher black level, an associated lowercontrast ratio, and uneven luminance.

(2) Alignment Angle

In the IPS-TFT-LCD, the initial alignment direction needs to be set inprinciple at a certain angle or more shifted from the direction in whichthe electrode extends or the direction perpendicular thereto. Theelectrode refers to a signal wiring electrode, a common electrode inpixels, and a pixel electrode. The definition of the initial alignmentdirection through the rubbing requires the fibers of approximately 10 to30 microns to rub in a predetermined angular direction as describedabove. However, the step of the wire such as the signal wiringelectrode, the common electrode in pixels, or the pixel electrodeextending in a certain direction at their ends draws the fibers towardthe step from the set angle to produce misalignment, thereby reducingimage quality such as a higher black level.

(3) Expression of Dark Level

One of the characteristics of the IPS-TFT-LCD is excellent expression ofa dark level (black display). Thus, misalignment is easily noticeable ascompared with the other schemes. In the conventionally normally opentype TN scheme, the dark level is provided while a high voltage isapplied. In this case, most of liquid crystal molecules align in thedirection of the electric field which is one direction perpendicular tothe substrate surface at a high voltage, and the dark level is providedfrom the relationship between the arrangement of the liquid crystalmolecules and the placement of a polarizing plate. Thus, the uniformityof the dark level hardly depends on the initial alignment state at a lowvoltage in principle. In addition, since human eyes recognize unevenluminance as a relative ratio of luminance and make response close to alogarithmic scale, they are sensitive to variations in the dark level.From this viewpoint, the conventional normally open type TN scheme inwhich the liquid crystal molecules are forcedly arranged in onedirection at a high voltage is advantageous in that it is not sensitiveto the initial alignment state.

On the other hand, in the IPS scheme, display of a dark level isperformed at a low voltage or no voltage, so that it is sensitive todisturbance of the initial alignment state. In particular, whenhomogeneous alignment is used in which the alignment directions ofliquid crystal molecules are in parallel with each other on an uppersubstrate and a lower substrate, and the light transmission axis of oneof polarizing plates is set in parallel with the alignment direction ofthe liquid crystal molecules and the light transmission axis of otherpolarizing plate is set orthogonally thereto (called a birefringencemode), polarized light incident on the liquid crystal layer istransmitted with almost no disturbance of lineal polarization. This iseffective in providing excellent expression of the dark level.

The transmittance T in the birefringence mode is expressed by thefollowing equation:

T=T ₀·sin² {2θ(E)}·sin² {(π·d _(eff) ·Δn)/λ}

where T₀ represents a coefficient which is a numerical value determinedmainly by the transmittance of the polarizing plate for use in theliquid crystal panel, θ(E) represents an angle between the alignmentdirection of liquid molecules (the effective optical axis of the liquidcrystal layer) and the polarized light transmission axis, E representsan applied electric field intensity, d_(eff) represent the effectivethickness of the liquid crystal layer, Δn represents the refractiveindex anisotropy of liquid crystal, and λ represents the wavelength oflight. The product of the effective thickness d_(eff) of the liquidcrystal layer and the refractive index anisotropy Δn of the liquidcrystal, that is, d_(eff)·Δn is called retardation. The thicknessd_(eff) of the liquid crystal layer does not refer to the thickness ofthe whole liquid crystal layer but corresponds to the thickness of theliquid crystal layer which actually changes in the alignment directionwhen a voltage is applied thereto. This is because the liquid crystalmolecules near the interface of the liquid crystal layer do not changein the alignment direction due to the influence of anchoring at theinterface even when a voltage is applied thereto. Thus, assuming thethickness of the whole liquid crystal layer sandwiched between thesubstrates is d_(LC), the relationship d_(eff)<d_(LC) is always foundbetween the thicknesses d_(LC) and d_(eff). The difference between themcan be estimated at approximately 20 nm to 40 nm, although it depends onthe liquid crystal material used in the liquid crystal panel and thetype of the interface in contact with the liquid crystal layer, forexample the material of the alignment film.

As apparent from the above equation, the term sin² {2θ(E)} depends onthe electric field intensity, and the luminance can be adjusted bychanging the angle θ in accordance with the electric field intensity E.For the normally close type, polarizing plates are set to satisfy θ=0when no voltage is applied, and it is sensitive to disturbance of theinitial alignment direction.

In this manner, the uniformity of alignment is a very important factorin the IPS scheme, and problems in the currently used rubbing techniquehave become apparent. In general, the rubbing alignment processingincludes many problems associated with the rubbing processing techniquesuch as TFT breakage due to static electricity produced by friction,unfavorable display due to misalignment from disordered fiber ends of arubbing cloth or dust, and the need for frequent exchanges of rubbingcloths. For the purpose of solving the problems associated with therubbing alignment processing, a so-called “rubbing-less” alignmenttechnique for aligning liquid crystal molecules without the rubbing hasbeen studied and various processes thereof have been proposed. Amongother things, a process has been proposed in which polarized ultravioletrays or the like are irradiated to the surface of a polymer film toalign liquid crystal molecules without the rubbing.

As an example, a process disclosed in “Non-Patent Document 1” ischaracterized in that it does not require the conventional rubbingprocessing and realizes the alignment of liquid crystal molecules in apredetermined direction through irradiation of polarized light. Theprocess is advantageous in presenting no problems such as damages on thefilm surface and static electricity associated with the rubbingtechnique and providing a simpler production process in view ofindustrial production. The process has attracted attention as a newliquid crystal alignment processing process without using the rubbingprocessing.

As a material of the liquid crystal alignment film used in the previousreports, the use of a polymer compound having a photoreactive group inthe side chain of a polymer has been proposed for the need to providephotochemical sensitivity to polarized light. A representative examplethereof is polyvinylcinnamate, in which case it is thought thatdimerization in the side chain through light irradiation developsanisotropy in a polymer film to align the liquid crystal. Anotherproposal involves dispersing low-molecular dichroic azo dye in a polymermaterial and irradiating a film surface with polarized light to allowthe alignment of liquid crystal molecules in a predetermined direction.In addition, the alignment of liquid crystal molecules achieved byirradiating a particular polyimide film with polarized ultraviolet raysor the like has been reported. In this case, it is contemplated that thelight irradiation decomposes the polyimide main chain in a certaindirection to develop the liquid crystal alignment.

-   Patent Document 1: JP-B-63-21907-   Patent Document 2: U.S. Pat. No. 4,345,249-   Patent Document 3: WO91/10936-   Patent Document 4: JP-A-6-22739-   Patent Document 5: JP-A-6-160878-   Patent Document 6: JP-A-9-73101-   Patent Document 7: Japanese Patent No. 3303766-   Patent Document 8: JP-A-11-218765-   Non-Patent Document 1: W. M. Gibbons et al., Nature, 351, 49 (1991)

DISCLOSURE OF THE INVENTION

In this manner, the photo-alignment process through light irradiationhas been proposed and studied as the rubbing-less alignment techniquefor solving the problems in the rubbing alignment technique, but it hasthe following problems from a practical standpoint. In a polymericmaterial obtained by introducing a photoreactive group in the side chainof a polymer represented by polyvinylcinnamate, the heat stability ofalignment is insufficient and satisfactory reliability is not ensuredfrom a practical viewpoint. In this case, since it is thought that theside chain of the polymer corresponds to the site of the structure whichdevelops the alignment of liquid crystal, it is difficult to say thatthe technique is preferable in providing more uniform alignment ofliquid crystal molecules and more resistant alignment. Whenlow-molecular dichroic dye is dispersed in a polymer, the dye itself foraligning liquid crystal is the low molecular substance, and from theviewpoint of practical use, problems remain in terms of reliability forheat and light.

In addition, in the process of irradiating particular polyimide withpolarized ultraviolet rays, the polyimide itself is reliable in heatresistance or the like, but it is thought that the alignment mechanismis caused by decomposition through the light, and it is thus difficultto ensure sufficient reliability for practical use. Specifically, whenthe liquid crystal alignment with the polarized light irradiation isapplied in the future, it is necessary not only to initially align theliquid crystal but also to develop more stable alignment from theviewpoint of reliability. In view of actual industrial application,selection of a thermally stable polymer structure is desired. From thosepoints, the polymer material proposed conventionally for the liquidcrystal alignment through light irradiation is not sufficient in thealignment property and the stability, which actually presents asignificant problem in realizing the rubbing-less alignment throughlight irradiation.

Thus, it is an object of the present invention to provide, particularly,a large-sized liquid crystal display capable of solving the inherentproblem of the small production margin in the alignment processing inthe IPS-TFT-LCD described above, reducing the occurrence of defectivedisplay due to variations in the initial alignment direction, realizingthe stable liquid crystal alignment, and having high image quality witha higher contrast ratio. It is another object of the present inventionto provide a process of producing a high-quality and high-definitionliquid crystal display with excellent mass productivity.

To achieve the abovementioned objects, the present invention provides aliquid crystal display comprising: a pair of substrates, at least one ofthe substrates being transparent; a liquid crystal layer disposedbetween the pair of substrates; a group of electrodes formed on one ofthe pair of substrates for applying an electric field having a componentsubstantially in parallel with a surface of the substrate to the liquidcrystal layer; a plurality of active devices connected to the group ofelectrodes; an alignment control film disposed between the liquidcrystal layer and at least one of the pair of substrates; and opticalmeans formed on at least one of the pair of substrates for changing theoptical property of the liquid crystal layer in accordance with analignment state of molecules of said liquid crystal layer, wherein atleast one of the alignment control films is an alignment control filmcomprising photoreactive polyimide and/or polyamic acid provided with analignment control ability by irradiation of substantially linearlypolarized light.

The present invention is characterized in that liquid crystal moleculesin the liquid crystal layer on the alignment control film have a longaxis in a direction orthogonal to a polarization axis of thesubstantially linearly polarized light for irradiation. In particular,it is desirable that the photoreactive alignment control film ispolyamic acid or polyimide comprising at leastcyclobutanetetracarboxylic acid dianhydride as acid anhydride and atleast aromatic diamine as diamine.

The present invention is characterized in that thecyclobutanetetracarboxylic acid dianhydride and its derivative are acompound represented by a formula [17]:

where R₁, R₂, R₃, R₄ each represent a hydrogen atom, a fluorine atom, analkyl group or alkoxyl group with a carbon number of 1 to 6.

On the other hand, the aromatic diamine compound contains at least oneof compounds selected from a group of compounds consisting of onesrepresented by formulas [18] to [32]:

where R₁, R₂, R₃, R₄ each represent a hydrogen atom, a fluorine atom, analkyl group or alkoxyl group with a carbon number of 1 to 6, or a vinylgroup {—(CH₂)_(m)—CH═CH₂, m=0, 1, 2} or an acetyl group{—(CH₂)_(n)—C≡CH, n=0, 1, 2}, and in the formula [5], X represents abond group —S—, —CO—, —NH—.

When the alignment control film is formed as a thin film having athickness from 1 nm to 100 nm, the light transmittance is improved andthe efficiency of light reaction with polarized light irradiation iseffectively improved. In addition, when the liquid crystal display isproduced, the voltage for driving the liquid crystal is effectivelyapplied to the liquid crystal layer. Furthermore, when the alignmentcontrol film on the electrode is formed as a thin film having athickness from 1 nm to 50 nm, and even from 1 nm to 30 nm, it ispossible to reduce a direct current voltage component (a so-calledresidual DC voltage) remaining between the electrode/alignment controlfilm/liquid crystal layer/alignment control film/electrode in each pixelof the liquid crystal display, and after-image and persistencecharacteristics are effectively enhanced.

In addition, the present invention is characterized in that the liquidcrystal layer of the liquid crystal display has a pretilt angle equal toor smaller than one degree. In the conventional rubbing alignmenttechnique, the end of the electrode step acts as a guide for the fibersof a rubbing cloth to draw the fibers in the direction in which the stepextends, and the fibers do not extend to the corner of the step, whichmay prevent the alignment processing to cause misalignment. Inparticular, when at least one of a pixel electrode, a common electrode,and a common electrode wire is formed of a transparent electrode, thealignment state near the electrode step is easily recognized, and thepresent invention is effective. Especially, when the transparentelectrode is formed of an ion-doped titanium oxide film or an ion-dopedzinc oxide film (ZnO), the present invention effectively works. When thepixel electrode and the common electrode opposite thereto are arrangedin parallel with each other and formed of zigzag bending structures, theadhesion of the liquid crystal alignment film to an underlying organicinsulating film may be poor, and the conventional rubbing alignmentprocessing may cause defective display such as stripping of thealignment film. In such a case, the present invention is effective.

The present invention is particularly effective when the commonelectrode and/or the pixel electrode is formed on the organic insulatingfilm and the liquid crystal alignment film is formed on the organicinsulating film and the electrodes. In addition, the present inventionis characterized in that the liquid crystal molecules have substantiallythe same alignment control directions at two interfaces between theliquid crystal layer and the alignment control film formed on each ofthe paired substrates.

The present invention is characterized in that the liquid crystalalignment processing is performed by irradiating the liquid crystalalignment film with polarized light. The present invention ischaracterized in that the polarized light used in the alignmentprocessing has a wavelength range from 200 to 400 nm. In addition, thepresent invention is more effective when polarized light with at leasttwo wavelengths, that is, substantially linearly polarized light with afirst wavelength and light with a second wavelength are used in thealignment processing.

The present invention is also characterized in that the liquid crystalalignment film has a glass transition temperature equal to or higherthan 250° C. The present invention more effectively functions byapplying at least one processing of heating, irradiation of infraredrays, irradiation of far infrared rays, irradiation of electron beams,and irradiation of radioactive rays when the irradiation of the liquidcrystal alignment film with the polarized light is performed to providethe liquid crystal alignment ability. When the liquid crystal alignmentability is provided by irradiating the liquid crystal alignment filmwith the polarized light, the heating, irradiation of infrared rays,irradiation of far infrared rays, irradiation of electron beams, orirradiation of radioactive rays is performed to accelerate the provisionof the liquid crystal alignment ability through the polarized lightirradiation and induce cross-link reaction or the like, therebyeffectively promoting and stabilizing the liquid crystal alignmentability. Especially, at least one processing of heating, irradiation ofinfrared rays, irradiation of far infrared rays, irradiation of electronbeams, and irradiation of radioactive rays is performed to overlap intime with the irradiation of polarized light, so that the presentinvention more effectively functions.

The present invention effectively works by performing imidationcalcination processing of the alignment control film to overlap in timewith the irradiation of polarized light. In particular, when at leastone processing of heating, irradiation of infrared rays, irradiation offar infrared rays, irradiation of electron beams, and irradiation ofradioactive rays is performed in addition to the polarized lightirradiation of the liquid crystal alignment film, it is desirable to setthe temperature of the alignment control film in a range from 100 to400° C., and more preferably, from 150 to 300° C. It is possible andeffective that the processing of heating, irradiation of infrared rays,irradiation of far infrared rays also serves as the imidationcalcination (firing) processing.

In the present invention, the target contrast ratio is equal to orhigher than 500:1, and the target time for eliminating after-image isequal to or shorter than five minutes. The time for eliminatingafter-image is determined by a process defined in the followingembodiments.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a section view showing a pixel portion for explaining thepixel structure of Embodiment 1 of a liquid crystal display according tothe present invention.

FIG. 2 a is a plan view and FIGS. 2 b and 2 c are sectional views,respectively, showing a pixel portion for explaining the pixel structureof Embodiment 1 of the liquid crystal display according to the presentinvention.

FIG. 3 is a section view showing a pixel portion for explaining thepixel structure of Embodiment 2 of the liquid crystal display accordingto the present invention.

FIG. 4 a is a plan view and FIGS. 4 b and 4 c are sectional views,respectively, showing a pixel portion for explaining the pixel structureof the liquid crystal display which is Embodiment 2 of the liquidcrystal display according to the present invention.

FIG. 5 is a section view showing the structure of a pixel of the liquidcrystal display for explaining Example of the present invention.

FIG. 6 is a section view showing the structure of a pixel of the liquidcrystal display for explaining Example of the present invention.

FIG. 7 is a section view showing a pixel portion for explaining thepixel structure of the liquid crystal display which is Embodiment 4 ofthe liquid crystal display according to the present invention.

FIG. 8 is a plan view showing a pixel portion for explaining the pixelstructure of the liquid crystal display which is Embodiment 4 of theliquid crystal display according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will hereinafter be described indetail with reference to the drawings. In the following, a substratewhich has active devices such as thin film transistors formed thereon isreferred to as an active matrix substrate. When an opposite substratehas a color filter thereon, this is referred to as a color filtersubstrate.

FIG. 1 is a schematic section view showing almost one pixel forexplaining Embodiment 1 of a liquid crystal display according to thepresent invention. FIGS. 2( a) to 2(c) are schematic diagrams of theactive matrix substrate for explaining the structure of almost one pixelfor describing Embodiment 1 of the liquid crystal display according tothe present invention, in which FIG. 2( a) is a plan view, FIG. 2( b) isa section view taken along an A-A′ line of FIG. 2( a), and FIG. 2( c) isa section view taken along a B-B′ line of FIG. 2( a). FIG. 1 correspondsto part of the section view taken along the A-A′ line of FIG. 2( a). Thesection views of FIG. 2( b) and FIG. 2( c) schematically show theemphasized structures of main portions and do not correspond exactly tothe cut portions along the A-A′ line and the B-B′ line of FIG. 2( a),respectively. For example, FIG. 2( b) does not show a semiconductor film116, and FIG. 2( c) representatively shows only part of a through-holefor connecting an opposite electrode with a common wire 120.

In the liquid crystal display device of Embodiment 1, a gate electrode(scanning signal electrode) 104 made of Cr (chromium) and the commonwire (common electrode wire) 120 are disposed on a glass substrate 101serving as the active matrix substrate. A gate insulating film 107 madeof silicon nitride is formed to cover the gate electrode 104 and thecommon electrode wire 120. The semiconductor film 116 made of amorphoussilicon or polysilicon is disposed above the gate electrode 104 throughthe gate insulating film 107 to function as an active layer of a thinfilm transistor (TFT) as an active device. A drain electrode (videosignal wire) 106 made of CrMo (Chromium/Molybdenum) and a sourceelectrode (pixel electrode) 105 are disposed to partly lie on thepattern of the semiconductor film 116. A protecting film 108 made ofsilicon nitride is formed to cover all of them.

In addition, as schematically shown in FIG. 2( c), a common electrode103 connected to the common electrode wire 120 through a through-hole103′ formed to extend through the gate insulating film 107 and theprotecting film 108 is disposed on an overcoat layer 112. Also, asapparent from FIG. 2( a), the common electrode 103 is formed to be drawnfrom the common electrode wire 120 through the through-hole 103′ suchthat the common electrode 103 is opposite to the pixel electrode 105 inthe area of one pixel in a plane.

Thus, in Embodiment 1 of the present invention, the pixel electrode 105is disposed in a layer lower than the protecting film 108 in a layerlower than the organic protecting film 112 and the common electrode 103is disposed on the organic protecting film 112. One pixel is formed bythe plurality of areas sandwiched between the pixel electrode 105 andcommon electrode 103. An alignment control film 109 is formed on thesurface of the active matrix substrate on which unit pixels formed asdescribed above are arranged in matrix, that is, on the organicprotecting film 112 on which the common electrode 103 is formed.

On the other hand, as shown in FIG. 1, a color filter layer 111 isdisposed on the glass substrate 102 constituting the opposite substratesuch that it is divided by a light shield portion (black matrix) 113 foreach pixel. The color filter layer 111 and the light shield portion 113are covered with the organic protecting film 112 made of a transparentinsulating material. The alignment control film 109 is also formed onthe organic protecting film 112 to constitute the color filtersubstrate.

These alignment control films 109 are provided with a liquid crystalalignment ability through irradiation of linearly polarized light ofultraviolet rays taken with a pile polarizer of stacked quartz platesfrom a high pressure mercury lamp serving as a light source. Thealignment control film has a surface which is cross-linked throughheating or the like.

The glass substrate 101 constituting the active matrix substrate and theglass substrate 102 constituting the opposite electrode are disposedwith their surfaces on the alignment control films 109 opposite to eachother. A liquid crystal layer (liquid crystal composition layer) 110′comprised of liquid crystal molecules 110 is disposed between them. Apolarizing plate 114 is formed on each of the outer surfaces of theglass substrate 101 constituting the active matrix substrate and theglass substrate 102 constituting the opposite electrode.

As described above, an active matrix type liquid crystal display (thatis, a TFT liquid crystal display) using thin-film transistors is formed.In the TFT liquid crystal display, the liquid crystal molecules 110constituting the liquid composition layer 110′ are aligned substantiallyin parallel with the surfaces of the substrates 101 and 102 disposedopposite to each other when no voltage is applied thereto, andhomogeneously aligned in an initial alignment direction defined byphoto-alignment processing. When a voltage is applied to the gateelectrode 104 to turn on the thin film transistor (TFT), a potentialdifference between the pixel electrode 105 and the common electrode 103applies an electric field 117 to the liquid crystal composition layer.The interaction between the dielectric anisotropy of the liquid crystalcomposition and the electric field changes the orientation of the liquidcrystal molecules 110 constituting the liquid crystal composition layerto the electric field direction. At this point, the refractive indexanisotropy of the liquid crystal composition layer and the effect of thepolarizing plates 114 can change the light transmittance of the liquidcrystal display to realize display.

The organic protecting film 112 may be formed by using a thermosettingresin such as an acrylic resin, an epoxy acrylic resin, or a polyimideresin with high insulation and transparency. The organic protecting film112 may be formed by using a photo-curing transparent resin or aninorganic material such as a polysiloxane resin. Alternatively, theorganic protecting film 112 may also serve as the alignment control film109.

As described above, according to Embodiment 1, the liquid crystalalignment control ability of the alignment control film 109 is providednot by using the rubbing alignment processing through direct frictionwith a buff cloth but by using a non-contact photo-alignment process. Itis thus possible to provide uniform alignment over the entire displayarea without local misalignment near the electrodes.

Next, Embodiment 2 of the liquid crystal display according to thepresent invention will be described. FIG. 3 is a schematic section viewshowing almost one pixel for explaining Embodiment 2 of the liquidcrystal display according to the present invention. FIGS. 4( a) to 4(c)are schematic diagrams of an active matrix substrate for explaining thestructure of almost one pixel for describing Embodiment 2 of the liquidcrystal display according to the present invention, in which FIG. 4( a)is a plan view, FIG. 4( b) is a section view taken along an A-A′ line ofFIG. 3( a), and FIG. 4( c) is a section view taken along a B-B′ line ofFIG. 3( a). FIG. 3 shows part of the section view taken along the A-A′line of FIG. 4( a). The section views of FIG. 4( b) and FIG. 4( c)schematically show the emphasized structures of main portions and do notcorrespond exactly to the cut portions along the A-A′ line and the B-B′line of FIG. 2( a), respectively. For example, FIG. 2( b) does not showa semiconductor film 116.

In the liquid crystal display device of Embodiment 2 of the presentinvention, a gate electrode 104 made of Cr and a common electrode wire120 are disposed on a glass substrate 101 constituting an active matrixsubstrate. A gate insulating film 107 made of silicon nitride is formedto cover the gate electrode 104 and the common electrode wire 120. Thesemiconductor film 116 made of amorphous silicon or polysilicon isdisposed above the gate electrode 104 through the gate insulating film107 to function as an active layer of a thin film transistor (TFT)serving as an active device.

A drain electrode 106 made of Chromium/Molybdenum and a source electrode(pixel electrode) 105 are disposed to partly lie on the pattern of thesemiconductor film 116. A protecting film 108 made of silicon nitride isformed to cover all of them. An organic protecting film 112 is disposedon the protecting film 108. The organic protecting film 112 is formed ofa transparent material such as an acrylic resin. The pixel electrode 105is formed of a transparent electrode such as ITO (In₂O₃:Sn). A commonelectrode 103 is connected to the common electrode wire 120 through athrough-hole 103′ extending through the gate insulating film 107, theprotecting film 108, and the organic protecting film 112.

The common electrode 103 forming a pair with the pixel electrode 105when an electric field is applied for driving liquid crystal is formedto surround the area of one pixel in a plane. The common electrode 103is disposed on an overcoat layer 112 on the organic protecting film 112.The common electrode 103 is disposed to hide the drain electrode 106,the scanning signal wire 104, and the thin-film transistor (TFT) servingas the active device arranged in a lower layer when viewed from above,and also serves as a light shield layer for shielding the semiconductorfilm 116 from light.

An alignment control film 109 is formed on the surface of the glasssubstrate 101 constituting the active matrix substrate on which unitpixels (one pixel) formed as described above are arranged in matrix,that is, on the organic protecting film 112 and on the common electrode103 formed thereon. On the other hand, on a glass 102 constituting theopposite substrate, a color filter layer 111, the organic protectingfilm 112 formed thereon, and the alignment control film 109 are formed.

Similarly to Embodiment 2, the alignment control films 109 are providedwith a liquid crystal alignment ability through irradiation of linearlypolarized light of ultra-violet rays taken with a pile polarizer ofstacked quartz plates from a high pressure mercury lamp serving as alight source. The alignment control film has a surface which iscross-linked through heating or the like.

The glass substrate 101 and the opposite substrate 102 are disposed withtheir surfaces having the alignment control films 109 formed thereonopposite to each other. A liquid crystal composition layer 110′comprised of liquid crystal molecules 110 is disposed between them. Apolarizing plate 114 is formed on each of the outer surface of the glasssubstrate 101 and the opposite substrate 102.

As described above, in Embodiment 2 of the present invention, similarlyto the abovementioned Embodiment 1, the pixel electrode 105 is disposedin a layer lower than the organic protecting film 112 and the protectingfilm 108, and the common electrode 103 is disposed above the pixelelectrode 105 and the organic protecting film 112. When the commonelectrode 103 has a sufficiently low electric resistance, the commonelectrode 103 can also serve as the common electrode wire 120 formed inthe lowermost layer. In this case, the formation of the common electrodewire 120 disposed in the lowermost layer and the associated processingof the through-hole can be omitted.

In Embodiment 2, one pixel is formed by the area surrounded by thecommon electrode 103 formed in a lattice form as shown in FIG. 4( a).The common electrode 103 is arranged to divide one pixel into four areasin combination with the pixel electrode 105. The pixel electrode 105 andthe opposite common electrode 103 are formed of zigzag bendingstructures arranged in parallel with each other, and one pixel providestwo or more sub-pixels. This results in a structure which cancels a huechange in a plane.

FIG. 5 is a schematic section view showing almost one pixel forexplaining Embodiment 3 of the liquid crystal display according to thepresent invention. In FIG. 5, the same reference numerals as those inthe figures of the abovementioned respective embodiments correspond tothe same functional portions. As shown in FIG. 5, in Embodiment 3, apixel electrode 105 disposed in a layer lower than a protecting film 108is pulled up on an organic protecting film 112 through a through-hole103′ and placed in the same layer as a common electrode 103. When thisstructure is used, a voltage for driving a liquid crystal can be reducedfurther.

In the TFT liquid crystal display configured as described above, liquidcrystal molecules 110 constituting a liquid crystal composition layer110′ are substantially in parallel with the surfaces of glass substrates101 and 102 disposed opposite to each other when no voltage is applied,and homogeneously aligned in an initial alignment direction defined byphoto-alignment processing. When a voltage is applied to a gateelectrode 104 to turn on a thin film transistor (TFT), a potentialdifference between the pixel electrode 105 and the common electrode 103applies an electric field 117 to the liquid crystal composition layer110′. The interaction between the dielectric anisotropy of the liquidcrystal composition and the electric field changes the orientation ofthe liquid crystal molecules 110 to the electric field direction. Atthis point, the refractive index anisotropy of the liquid crystalcomposition layer 110′ and the effect of polarizing plates 114 canchange the light transmittance of the liquid crystal display to realizedisplay.

In the abovementioned respective embodiments of the present invention,it is possible to provide a plurality of sets of display areas definedby the common electrode and the pixel electrode in one pixel. Since theplurality of sets formed in this manner can reduce the distance betweenthe pixel electrode and the common electrode even when one pixel has alarge size, the voltage applied to drive the liquid crystal can bereduced.

In the abovementioned respective embodiments of the present invention,no particular limitations are imposed on the material of the transparentconductive film forming at least one of the pixel electrode and thecommon electrode. However, in consideration of easy processing,reliability and the like, it is desirable to use a transparentconductive film of an ion-doped titanium oxide such as ITO(Indium-Tin-Oxide) or an ion-doped zinc oxide.

Generally, in the IPS scheme, it is known that an interface tilt withrespect to the substrate is not required in principle unlikely thevertical electric field scheme represented by the conventional TNscheme, and more favorable viewing characteristics are achieved with asmaller interface tilt angle. A small interface tilt angle is alsodesirable in the light alignment control film, and specifically, anangle of one degree or smaller is effective.

Next, description will be made for the formation of the alignmentcontrol film by using the rubbing-less alignment technique for theliquid crystal alignment control film as the production process of theliquid crystal display according to the present invention. The flow ofthe formation process of the alignment control film according to thepresent invention is summarized as follows:

1. Applying and forming the alignment control film (forming a uniformlyapplied film over the entire display area), then

2. Firing for imidation of the alignment control film (removing avarnish solvent and promoting polyimidation with high heat resistance),then

3. Providing the liquid crystal alignment ability through irradiation ofpolarized light (providing a uniform alignment ability for the displayarea), and

4. Promoting and stabilizing the alignment ability by (heating,irradiation of infrared rays, irradiation of far infrared rays,irradiation of electron beams, irradiation of radioactive rays).

The alignment control film is formed through the above four-stepprocess. In terms of the order of the steps 1 to 4, additional effectscan be expected in the case described below.

(1) The steps 3 and 4 are processed with temporal overlap to acceleratethe provision of the liquid crystal alignment ability and inducecross-link reaction and the like, allowing more effective formation ofthe alignment control film. In particular, when polyimide usingcyclobutanetetracarboxylic acid dianhydride is used for the alignmentcontrol film, it is thought that the alignment control ability isprovided by cleavage of the cyclobutane ring with the irradiation ofpolarized light. When the heating step is performed to overlap in timewith the irradiation of polarized light, (i) maleimide is produced fromthe cleavage of the ring. The surface cross-linked by the maleimide isstabilized to provide a contrast and reduced after-image. (ii) Polymeris split. The low molecular part remains. Since the low molecular partis produced, it is vulnerable to stress.

The surface of the unstable alignment control film due to thecarbon-carbon double bond can be stabilized by the cross-link reaction.

(2) When the heating, the irradiation of infrared rays, the irradiationof far infrared rays or the like in the step 4 is used, theabovementioned steps 2, 3, and 4 are performed to overlap in time toallow the step 4 to serve also as the imidation step 2, thereby formingthe alignment control film in a short time period.

Next, specific examples of the production process of the liquid crystaldisplay according to the present invention will be described.

Example 1

Example 1 corresponds to the liquid crystal display described inEmbodiment 1 of the present invention explained above. Detaileddescription will hereinafter be made for Example 1 of the presentinvention with reference to FIGS. 1 and 2.

In producing the liquid crystal display which is Example 1 of thepresent invention, a glass substrate with a thickness of 0.7 mm andhaving polished surfaces is used as the glass substrate 101 constitutingthe active matrix substrate and the glass substrate 102 constituting theopposite substrate (color filter substrate). A thin film transistor 115formed above the glass substrate 101 is comprised of the pixel electrode105, the signal electrode 106, the scanning electrode 104, and theamorphous silicon 116. All of the scanning electrode 104, the commonelectrode wire 120, the signal electrode 106, and the pixel electrode105 were formed by patterning a chromium film. The interval between thepixel electrode 105 and the common electrode 103 was set to 7 μm. Whilethe chromium film which had low resistance and allowed easy patterningwas used for the common electrode 103 and the pixel electrode 105, anITO film may be used to form a transparent electrode to achieve a betterluminance characteristic. The gate insulating film 107 and theprotecting insulating film 108 were formed of silicon nitride and eachhad a thickness of 0.3 μm. An acrylic resin was applied thereon, andheating processing was performed at 220° C. for an hour to form thetransparent and insulating organic protecting film 112.

Next, photolithography with etch processing was performed to form thethrough-hole reaching the common electrode wire 120 as shown in FIG. 2(c) to form the common electrode 103 connected with the common electrodewire 120 through patterning.

The resulting structure had the pixel electrode 105 disposed between thethree common electrodes 103 in the unit pixel (one pixel) as shown inFIG. 2( a), thereby forming the active matrix substrate with the numberof pixels of 1024×3×768 constituted by 1024×3 (for R, G, and B) signalelectrodes 106 and 768 scanning electrodes 104.

Next, as the alignment control film, polyamic acid varnish consisting of4,4′ diaminostilbene shown in a formula [33] and1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shown in a formula[34] was prepared as 5% by weight for the concentration of the resin,40% by weight of NMP, 40% by weight of γ-butyrolactone, 15% by weight ofbutyl cellosolve, and formed through printing on the abovementionedactive matrix substrate. Heat treatment was performed at 220° C. for 30minutes for imidation to form the fine polyimide alignment control film109 of approximately 70 nm.

In the same manner, similar polyamic acid varnish was formed throughprinting on the surface of the other glass substrate 102 on which theITO was deposited, and was subjected to heat treatment at 220° C. for 30minutes to form the alignment control film 109 made of a fine polyimidefilm of approximately 70 nm.

While the substrate was heated to 200° C. by a hot plate, the polyimidealignment control film 109 was irradiated with polarized UV(ultraviolet) light in order to provide the liquid crystal alignmentability for the surface. A high pressure mercury lamp was used as alight source. The UV light of 240 nm to 380 nm was taken through aninterference filter and changed by a pile polarizer of stacked quartzsubstrates into linearly polarized light with a polarization ratio ofapproximately 10:1 before irradiation with an irradiation energy ofapproximately 5 J/cm². As a result, the alignment direction of theliquid crystal molecules on the surface of the alignment control filmwas found orthogonal to the polarization direction of the irradiatedpolarized UV light.

Next, the two glass substrates 101 and 102 were disposed such that therespective surfaces having the alignment control films 109 with theliquid crystal alignment ability were opposite to each other and aspacer consisting of dispersed spherical polymer beads was interposedbetween them. A seal agent was applied to peripheral portions. In thismanner, the liquid crystal display panel (referred also to as a cell)which was to serve as the liquid crystal display was assembled. Theliquid crystal alignment directions on the two glass substrates weresubstantially in parallel with each other and formed an angle of 75°with respect to the direction of an applied electric field. The cell wasinjected in vacuum with a nematic liquid crystal composition A with adielectric anisotropy Δ∈ of a positive value of 10.2 (1 kHz, 20° C.), arefractive index anisotropy Δn of 0.075 (a wavelength of 590 nm, 20°C.), a twist elastic constant K2 of 7.0 pN, a nematic-isotropic phasetransition temperature T (N−1) of approximately 76° C. and it was sealedby a sealing material made of an ultraviolet curing resin. The liquidcrystal panel was produced with the liquid crystal layer having athickness (gap) of 4.2 μm.

The liquid crystal display panel has a retardation (Δnd) ofapproximately 0.31 μm. A liquid crystal display panel of homogeneousalignment was produced by using an alignment control film and a liquidcrystal composition equivalent to those used in this panel. Themeasurement of the pretilt angle of the liquid crystal with the crystalrotation technique showed approximately 0.2 degrees. The liquid crystaldisplay panel was sandwiched between the two polarizing plates 114 suchthat one of the polarizing plates has a polarized light transmissionaxis substantially in parallel with the abovementioned liquid crystalalignment direction, and the other has a polarized light transmissionaxis orthogonal thereto. Then, a drive circuit, a backlight and the likewere connected to form a module, thereby providing the active matrixtype liquid crystal display. Example 1 employed the normally closecharacteristic in which dark display is produced at low voltage, whilebright display is produced at high voltage.

Then, when the display quality of the above-mentioned liquid crystaldisplay which is Example 1 of the present invention was evaluated, highquality display with a contrast ratio of 600:1 was observed and a wideviewing angle at halftone display was observed.

Then, evaluation was performed by using an oscilloscope in combinationwith a photodiode in order to quantitatively measure image persistenceand after-image in the liquid crystal display which is Example 1 of thepresent invention. First, a window pattern was displayed on the screenat the maximum luminance for 30 minutes, and then the whole screen wasswitched to halftone display in which after-image was most recognizable,in this case the luminance was set to 10% of the maximum luminance toevaluate the time until the edge pattern of the window patterndisappeared as an after-image relaxation time. It should be noted thatthe permissible after-image relaxation time is equal to or shorter thanfive minutes. As a result, the after-image relaxation time was oneminute or shorter in a range of operating temperatures (0 to 50° C.).Also, in a visual image quality after-image test, an excellent displaycharacteristic was found without any uneven display due to imagepersistence or after-image.

Conventionally, the photo-alignment can provide the alignment of liquidcrystal, but it is said that anchoring energy, that is, energy forbinding the aligned liquid crystal molecules to the surface of thealignment film is lower than that in the typical rubbing alignment. Itis said that the low anchoring energy causes insufficient reliability ofthe liquid crystal display as a product. In particular, it is said thatanchoring energy in an azimuth direction is more important thananchoring energy in a polar angle direction for the homogeneousalignment.

Thus, the same alignment film material as that in the liquid crystaldisplay provided in this manner was used to form an alignment film on aglass substrate through the same process and alignment processing wasperformed. The same liquid crystal composition was filled to form aliquid crystal cell. A torque balance technique (Hasegawa et al,Japanese Liquid Crystal Society Conference Proceedings 3B12 (2001) p251)was used to measure the strength of twist binding between liquid crystalmolecules at the interface and the surface of the alignment film, theanchoring energy A2 in the azimuth direction, and the energy was6.0×10⁻⁴ N/m.

Example 2

In the same manner as in Example 1 except the alignment control filmused in Example 2, polyamic acid comprising1,3-dimethyl-1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shownin a formula [35] as acid dianhydride and m-phenylenediamine shown in aformula [36] as a diamine compound was formed through printing on thesubstrate surface and subjected to firing at 230° C. for 30 minutes forimidation, thereby depositing the film with a thickness of approximately50 nm. Then, while the substrate was heated to 200° C. by a hot plate,the surface was subjected to photo-alignment processing through lightirradiation with polarized UV of krF excimer laser with a wavelength of248 nm and nitrogen laser with a wavelength of 337 nm.

Thereafter, a nematic liquid crystal composition A was filled in thesame manner as in Example 1. Annealing was then performed at 100° C. for10 minutes. Thus, favorable liquid crystal alignment was provided in adirection substantially perpendicular to the abovementioned polarizationdirection of the irradiation.

In this manner, the liquid crystal panel having the liquid crystal layerwith a thickness d of 4.0 μm was provided. A liquid crystal displaypanel of homogeneous alignment was produced by using an alignmentcontrol film and a liquid crystal composition equivalent to those usedin this liquid crystal panel. The measurement of the pretilt angle ofthe liquid crystal with the crystal rotation technique showedapproximately 0.5 degrees.

Then, when the display quality of the liquid crystal display wasevaluated in the same manner as in Example 1, and high quality displaywas observed with a contrast ratio of more than 500:1 over the entirescreen, which is substantially equivalent to that of the liquid crystaldisplay of Example 1, and a wide viewing angle was observed at halftonedisplay. In the same manner as in Example 1, the image persistence andafter-image relaxation time of the liquid crystal display werequantitatively estimated, and the after-image relaxation time wasapproximately one minute in a range of operating temperatures from 0 to50° C. Also, in the visual image quality after-image test, an excellentdisplay characteristic equivalent to that of Example 1 was found withoutany uneven display due to image persistence or after-image.

Comparative Example 1

As a Comparative Example 1 for explaining the effect of Example 2, aliquid crystal display panel was formed in the same manner as in Example1 except the alignment control film by using polyamic acid varnishconsisting of pyromellitic acid dianhydride shown in a formula [37] asacid dianhydride and p-phenylenediamine shown in a formula [38] as adiamine compound.

in the same manner as in Example 1, a wide viewing angle substantiallyequivalent to that of the liquid crystal display of Example 1 wasobserved, although the display with a contrast ratio of lower than 100:1over the entire screen was observed. Similarly to Example 1, the imagepersistence and after-image relaxation time of the liquid crystaldisplay were quantitatively evaluated, and the after-image relaxationtime was found approximately seven minutes in a range of operatingtemperatures from 0 to 50° C. Also, in the visual image qualityafter-image test, the after-image relaxation time was long, and anexcellent display characteristic equivalent to Example 1 was notachieved.

The value of A2 was approximately 6.5×10⁻⁴ N/m.

Example 3

A liquid crystal display panel was produced in the same manner as inExample 1 except the alignment control film by preparing polyamic acidvarnish using 1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shownin a formula [39] and pyromellitic acid dianhydride shown in a formula[40] as acid dianhydride with a mole fraction of 6:4 andp-phenylenediamine shown in a formula [41] as a diamine compound. Inthis event, the thickness of the alignment control film was set toapproximately 50 nm.

Then, when the display quality of the liquid crystal display wasevaluated in the same manner as in of Example 1, high quality displaywith a contrast ratio of more than 500:1 was observed over the entirescreen, which is substantially equivalent to that of the liquid crystaldisplay of Example 1, and a wide viewing angle at halftone display wasalso observed. In addition, when the image persistence and after-imagerelaxation time of the liquid crystal display were quantitativelyevaluated similarly to Example 1, the after-image relaxation time wasapproximately one minute in a range of operating temperatures from 0 to50° C. Also, in the visual image quality after-image test, an excellentdisplay characteristic equivalent to that in Example 1 was found withoutany uneven display due to image persistence or after-image.

The value of A2 was approximately 8.2×10⁻⁴ N/m.

In addition, two kinds of polyamic acid varnish were prepared by settingthe composition ratios of 1:1 and 4:6 for the abovementioned two kindsof acid dianhydride in the polyamic acid varnish used in the alignmentcontrol film, that is, 1,2,3,4-cyclobutanetetracarboxylic aciddianhydride and pyromellitic acid dianhydride. The respective kinds ofvarnish were used to produce two kinds of liquid crystal display panels.The contrast ratios of liquid crystal displays with the liquid crystaldisplay panels were approximately 470:1 and 200:1, respectively. Theafter-image relaxation times were approximately 2 and 6 minutes,respectively. The display characteristic was significantly degraded inthe composition ratio of 4:6 of 1,2,3,4-cyclobutanetetracarboxylic aciddianhydride and pyromellitic acid dianhydride as compared with the othercase.

The value of A2 of the panel with the contrast ratio of 200:1 wasapproximately 2.3×10⁻⁴ N/m.

Comparative Example 2

As a comparative example for explaining the effect of Example 3, aliquid crystal display panel was formed in the same manner as in Example1 except the alignment processing.

The alignment processing was performed as follows.

As an alignment control film, polyamic acid varnish consisting of 4,4′diaminostilbene shown in a formula [33] and1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shown in a formula[34] was prepared as 5% by weight for the concentration of the resin,40% by weight of NMP, 40% by weight of γ-butyrolactone, 15% by weight ofbutyl cellosolve, and formed through printing on the abovementionedactive matrix substrate. Heat treatment was performed at 220° C. for 30minutes for imidation to form the fine polyimide alignment control film109 of approximately 70 nm.

In the same manner, similar polyamic acid varnish was formed throughprinting on the surface of the other glass substrate 102 on which theITO was deposited and heat treatment was performed for 220° C. for 30minutes to form the alignment control film 109 made of a fine polyimidefilm of approximately 100 nm.

To provide the surface with the liquid crystal alignment ability, thepolyimide alignment control film 109 was irradiated with polarized UV(ultraviolet) light. A high pressure mercury lamp was used as a lightsource. The UV light in a range from 240 nm to 380 nm was taken throughan interference filter and changed by a pile polarizer of stacked quartzsubstrates into linearly polarized light with a polarization ratio ofapproximately 10:1 before irradiation with an irradiation energy ofapproximately 3 J/cm². However, at the irradiation of the polarizedlight, a step associated with cross-link such as heating was notperformed. As a result, the alignment direction of the liquid crystalmolecules on the surface of the alignment control film was foundperpendicular to the polarization direction of the irradiated polarizedUV light.

When the display quality thereof was evaluated in the same manner as inExample 1, a wide viewing angle substantially equivalent to that of theliquid crystal display of Example 1 was observed, but the display with acontrast ratio of lower than 100:1 over the entire screen was observed.In addition, when the image persistence and after-image relaxation timeof the liquid crystal display were quantitatively evaluated similarly toExample 1, the after-image relaxation time was approximately five minutein a range of operating temperatures from 0 to 50° C. The after-imagerelaxation time was also long in the visual image-quality after-imagetest. Thus, an excellent display characteristic equivalent to that inExample 1 was not achieved.

The value of A2 was approximately 0.5×10⁻⁴ N/m.

Comparative Example 3

As a comparative example for explaining the effect of Example 3, aliquid crystal display panel was formed in the same manner as in Example1 except the alignment processing.

As the alignment film, polyamic acid varnish consisting of pyromelliticacid dianhydride shown in a formula [37] as acid dianhydride andp-phenylenediamine shown in a formula [38] as a diamine compound wasused.

In the same manner, similar polyamic acid was formed through printing onthe surface of the other glass substrate 102 and heat treatment wasperformed at 220° C. for 30 minutes to form the alignment control film109 made of a fine polyimide film of approximately 70 nm.

To provide the surface with the liquid crystal alignment ability, thepolyimide alignment control film 109 was irradiated with polarized UV(ultraviolet) light while the substrate was hated by a hot plate at 200°C. A high pressure mercury lamp was used as a light source. The UV lightin a range from 240 nm to 380 nm was taken through an interferencefilter and changed by a pile polarizer of stacked quartz substrates intolinearly polarized light with a polarization ratio of approximately 10:1before irradiation with an irradiation energy of approximately 5 J/cm².As a result, the alignment direction of the liquid crystal molecules onthe surface of the alignment control film was found perpendicular to thepolarization direction of the irradiated polarized UV light.

When the display quality thereof was evaluated in the same manner as inExample 1, a wide viewing angle substantially equivalent to that of theliquid crystal display of Example 1 was observed, but the display with acontrast ratio of lower than 200:1 over the entire screen was observed.In addition, when the image persistence and after-image relaxation timeof the liquid crystal display were quantitatively evaluated similarly toExample 1, the after-image relaxation time was approximately five minutein a range of operating temperatures from 0 to 50° C. The after-imagerelaxation time was also long in the visual image-quality after-imagetest. Thus, an excellent display characteristic equivalent to that inExample 1 was not achieved.

The value of A2 was approximately 0.1×10⁻⁴ N/m.

Example 4

Next, Example 4 will be described with reference to FIGS. 3 and 4 as aspecific structure of the liquid crystal display which is Embodiment 2of the present invention. In the structure of the liquid crystal displaywhich is Example 4 of the present invention, a glass substrate with athickness of 0.7 mm and having polished surfaces is used as the glasssubstrates 101 and 102. The thin film transistor 115 is comprised of thepixel electrode 105, the signal electrode 106, the scanning electrode104, and the amorphous silicon 116. The scanning electrode 104 wasformed by patterning an aluminum film, the common electrode wire 120 andthe signal electrode 106 were formed by patterning a chromium film, andthe pixel electrode 105 was formed by patterning an ITO film. As shownin FIG. 4( a), the electrodes except the scanning electrode 104 wereformed in a zigzag bending electrode wiring pattern. The angle of thebending was set to 10 degrees. The gate insulating film 107 and theprotecting insulating film 108 were made of silicon nitride, and eachhad a thickness of 0.3 μm.

Next, photolithography with etch processing was performed to form thecylindrical through-hole reaching the common electrode wire 120 with adiameter of approximately 10 μm as shown in FIG. 4( c), and an acrylicresin was applied thereon. Heat treatment was performed thereon at 220°C. for an hour to form the transparent and insulating interlayerinsulating film 112 with a dielectric constant of approximately 4 and athickness of approximately 1 μm. The interlayer insulating film 112 wasused to planarize irregularities due to the steps of the pixel electrode105 in the display area and the steps in the boundary of the colorfilter layer 111 between adjacent pixels.

Then, the thorough-hole portion was again subjected to etching to adiameter of approximately 7 μm. The common electrode 103 connected withthe common electrode wire 120 was formed thereon by patterning an ITOfilm. At this point, the interval between the pixel electrode 105 andthe common electrode 103 was set to 7 μm. The common electrode 103 wasformed in a lattice form to lie above and cover the video signal wire106, the scanning signal wire 104, and the thin film transistor 115 tosurround the pixel such that it also serves as the light shield layer.

As a result, in shown in FIG. 4( a), the pixel electrode 105 wasarranged between the three common electrodes 103 in the unit pixel,thereby forming the active matrix substrate with the number of pixels of1024×3×768 constituted by 1024×3 (for R, G, and B) signal electrodes 106and 768 scanning electrodes 104.

Next, as the alignment control film 109, polyamic acid varnishconsisting of 1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shownin a formula [42] and 1,4-diaminonaphthalene shown in a formula [43] wasused to produce an alignment control film with a thickness ofapproximately 40 nm. In the alignment processing process, polarizedlight UV similar to that in Example 1 was irradiated with an irradiationenergy of approximately 3 Jcm⁻². However, heating processing of thesubstrate having the alignment control film formed thereon was performedon a hot plate at approximately 150° C. simultaneously during theirradiation of the polarized UV.

such that the surfaces having the alignment control films were oppositeto each other and a spacer consisting of dispersed spherical polymerbeads was interposed between them. A seal agent was applied toperipheral portions. In this manner, the liquid crystal display panelwas assembled. The liquid crystal alignment directions on the two glasssubstrates were substantially in parallel with each other and formed anangle of 75° with respect to the direction of an applied electric field.The liquid crystal display panel was injected in vacuum with a nematicliquid crystal composition A with a dielectric anisotropy Δ∈ of apositive value of 10.2 (1 kHz, 20° C.), a refractive index anisotropy Δnof 0.075 (a wavelength of 590 nm, 20° C.), a twist elastic constant K2of 7.0 pN, a nematic-isotropic phase transition temperature T (N−1) ofapproximately 76° C. and it was sealed by a sealing material made of anultraviolet curing resin. The liquid crystal panel was produced with theliquid crystal layer having a thickness (gap) of 4.2 μm. The panel has aretardation (And) of approximately 0.31 μm.

A liquid crystal display panel of homogeneous alignment was produced byusing an alignment control film and a liquid crystal compositionequivalent to those used in this liquid crystal display panel. Themeasurement of the pretilt angle of the liquid crystal with the crystalrotation technique showed approximately 0.2 degrees. The panel wassandwiched between the two polarizing plates 114 such that one of thepolarizing plates has a polarized light transmission axis substantiallyin parallel with the abovementioned liquid crystal alignment direction,and the other has a polarized light transmission axis orthogonalthereto. Then, a drive circuit, a backlight and the like were connectedto form a module, thereby providing the active matrix type liquidcrystal display. Example 4 employed the normally close characteristic inwhich dark display is produced at low voltage, while bright display isproduced at high voltage.

Next, the display quality of the liquid crystal display which is Example4 of the present invention was estimated. The aperture ratio was higherthan that of the liquid crystal display of Example 1, high qualitydisplay with a contrast ratio of 600:1 was observed, and a wide viewingangle at halftone display was also observed. In addition, in the samemanner as in Example 1, the image persistence and after-image relaxationtime of the liquid crystal display were quantitatively evaluated, andthe after-image relaxation time was found approximately one minute in arange of operating temperatures from 0 to 50° C. Also, in the visualimage quality after-image test, an excellent display characteristicequivalent to that of Example 1 was found without any uneven display dueto image persistence or after-image.

In addition, when an alignment control film formed on a glass substratein the same manner as in Example 4 was shaved to evaluate the glasstransition temperature of the alignment control film with DSC(Differential Scanning calorimetry), a clear glass transition pointcould not be observed in a range of temperatures from 50 to 300° C.Therefore, it is thought that the glass transition temperature of thealignment control film of Example 4 is more than 300° C. which is theupper limit of the measured temperature.

The value of A2 was approximately 8.6×10⁻⁴ N/m.

Example 5

As the alignment control film, polyamic acid varnish consisting of1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shown in a formula[44] and 2,6-diaminonaphthalene shown in a formula [45] was used toproduce an alignment control film with a thickness of approximately 50nm. In the alignment processing process, in the same manner as inExample 1, light from a high pressure mercury lamp was changed intopolarized UV with a wavelength range from 240 nm to 310 nm and apolarization ratio of 10:1 by using an interference filter and a quartzpile polarizer before irradiation with an irradiation energy ofapproximately 3 J/cm². The liquid crystal display panel of Example 5 wasproduced in the same manner as in Example 4 except the abovementionedpoints. When the display quality of the liquid crystal display providedby using the liquid crystal display panel was evaluated, high qualitydisplay equivalent to that of the liquid crystal display of Example 4was observed. A wide viewing angle at halftone display was alsoobserved.

In the same manner as in Example 1 of the present invention, the imagepersistence and after-image relaxation time of the liquid crystaldisplay of Example 5 were quantitatively evaluated, and the after-imagerelaxation time was found equal to or shorter than one minute in a rangeof operating temperatures from 0 to 45° C. similarly to Example 4. Also,in the visual image quality after-image test, an excellent displaycharacteristic was achieved without any uneven display due to imagepersistence or after-image. In addition, an alignment control filmformed on a glass substrate in the same manner as in Example 4 wasshaved to evaluate the glass transition temperature of the alignmentcontrol film with the DSC (Differential Scanning calorimetry), but aclear glass transition point could not be observed in a range oftemperatures from 50 to 300° C. Therefore, it is thought that the glasstransition temperature of the alignment control film of Example 5 ismore than 300° C. which is the upper limit of the measured temperature.

The value of A2 was approximately 6.8×10⁻⁴ N/m.

Example 6

Instead of the spacer made of polymer beads for use in the cell gapcontrol of the liquid crystal display, a negative photosensitive acrylicresin was applied, exposed, and developed for patterning into a columnwith a diameter of approximately 10 μm on the common electrode 103serving as the light shield layer in the layer higher than the scanningwire 104 near the TFT of each pixel before the formation of thealignment control film on the active matrix substrate. Then, as thealignment control film, polyamic acid varnish was prepared by using3,3′-dimethyl-4,4′-diaminobiphenyl shown in a formula (46) and4,4′-diaminophenylthioether shown in a formula (47) as diamine compoundswith a mole fraction of 1:2 and1,3-difluoro-1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shownin a formula (48) as acid anhydride and the film to form the film with athickness of approximately 30 nm. In the alignment processing process,in the same manner as in Example 5, light from a high pressure mercurylamp similar to that of Example 5 was changed into polarized UV with awavelength range from 240 nm to 310 nm and a polarization ratio of 10:1by using an interference filter and a quartz pile polarizer beforeirradiation with an irradiation energy of approximately 3 J/cm².Simultaneously, a soft x-ray generator was used to irradiate soft x-raysat short distance.

The liquid crystal display panel serving as Example 6 was produced withthe same procedure except the abovementioned steps. When the displayquality of the liquid crystal display which is Example 6 of the presentinvention was evaluated, high quality display with a higher contrastratio than that of the liquid crystal display of Example 5 was observed.A wide viewing angle was also observed at halftone display. It ispresumed that those results were achieved because of the completeelimination of light leakage which was caused by misalignment of theliquid crystal around the spacer beads distributed randomly in the pixelseen in the liquid crystal display of Example 5.

In the same manner as in Example 1 of the present invention, the imagepersistence and after-image relaxation time of the liquid crystaldisplay of Example 6 were quantitatively evaluated, and the after-imagerelaxation time was found equal to or shorter than one minute similarlyto Example 5. Also, in the visual image quality after-image test, anexcellent display characteristic was found without any uneven displaydue to image persistence or after-image.

The value of A2 was approximately 1.0×10⁻³ N/m.

Example 7

In the same manner as in Example 4 except the alignment control filmused and the alignment processing conditions, polyamic acid consistingof 9-methoxy-2,7-diaminofluorene as a diamine compound shown in aformula [49] and a 1,2,3,4-cyclobutanetetracarboxylic acid dianhydrideshown in a formula [50] as acid dianhydride was formed through printingon the substrate surface and fired at 230° C. for 30 minutes forimidation to deposit the film with a thickness of approximately 50 nm.Then, while the surface was irradiated with far infrared rays, polarizedUV of 337 nm from a nitrogen laser was irradiated with an irradiationenergy of approximately 3 J/cm² to perform photo-alignment processing.The temperature of the alignment control film at that point wasapproximately 180° C.

Thereafter, a nematic liquid crystal composition A was filled similarlyto Example 4. Annealing was performed at 100° C. for 10 minutes. Thus,favorable liquid crystal alignment was provided in a directionsubstantially perpendicular to the abovementioned polarization directionof the irradiation.

In this manner, the liquid crystal display having the liquid crystallayer with a thickness d of 4.0 μm was provided. A cell of homogeneousalignment was produced by using an alignment control film and a liquidcrystal composition equivalent to those used in this panel. Themeasurement of the pretilt angle of the liquid crystal with the crystalrotation technique showed approximately 0.3 degrees.

Then, when the display quality of the liquid crystal display which isExample 7 of the present invention was evaluated in the same manner asin Example 1, high quality display with a contrast ratio of more than600:1 over the entire screen was observed, which is equivalent to thatof the liquid crystal display of Example 1, and a wide viewing angle wasalso observed at halftone display. In the same manner as in Example 1 ofthe present invention, the image persistence and after-image relaxationtime of the liquid crystal display of Example 7 were quantitativelyevaluated, and the after-image relaxation time was found equal toshorter than one minute. Also, in the visual image quality after-imagetest, an excellent display characteristic was found without any unevendisplay due to image persistence or after-image.

The value of A2 was approximately 8.0×10⁻⁴ N/m.

Example 8

In the same manner as in Example 4 except the alignment control filmused and the alignment processing conditions, polyamic acid consistingof 2,7-diaminobiphenylene shown in a formula [51] as a diamine compoundand a 1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shown in aformula [52] as acid dianhydride was formed through printing on thesubstrate surface and fired at 230° C. for 30 minutes for imidation todeposit the film with a thickness of approximately 20 nm. Then, whilethe surface was irradiated with far infrared rays, polarized UV of 337nm from a nitrogen laser was irradiated to perform photo-alignmentprocessing through light irradiation. The temperature of the alignmentcontrol film at that point was approximately 200° C. Thereafter, anematic liquid crystal composition A was filled in the same manner as inExample 4. Annealing was then performed at 100° C. for 10 minutes. Thus,favorable liquid crystal alignment was provided in a directionsubstantially perpendicular to the abovementioned polarization directionof the irradiation.

In this manner, the liquid crystal display having the liquid crystallayer with a thickness d of 4.0 μm was provided. A cell of homogeneousalignment was produced by using an alignment control film and a liquidcrystal composition equivalent to those used in this panel. Themeasurement of the pretilt angle of the liquid crystal with the crystalrotation technique showed approximately 0.3 degrees.

Then, when the display quality of the above-mentioned liquid crystaldisplay which is Example 8 of the present invention was evaluated in thesame manner in Example 1, high quality display with a contrast ratio ofmore than 600:1 over the entire screen was observed, which is equivalentto that of the liquid crystal display of Example 4, and a wide viewingangle was also observed at halftone display. In the same manner as inExample 1 of the present invention, the image persistence andafter-image relaxation time of the liquid crystal display of Example 8were quantitatively evaluated, and the after-image relaxation time wasfound equal to or shorter than two minutes. Also, in the visual imagequality after-image test, an excellent display characteristic was foundwithout any uneven display due to image persistence or after-image.

In addition, it was found that the alignment control film used inExample 8 could also provide an excellent display characteristic asdescribed above, for example when light from a high pressure mercurylamp was changed into polarized UV of a wavelength range from 300 nm to380 nm through an interference filter and a quartz pile polarizer beforeirradiation with an irradiation energy of approximately 3 J/cm², inaddition to the combination of the irradiation of the far infrared raysand the irradiation of polarized UV from the nitrogen laser. Moreover,it was found that an excellent display characteristic similar to thatdescribed above could also be provided when the abovementioned polarizedUV of 300 to 380 nm was irradiated while carbon dioxide gas laser of10.5 μm and 200 mJ was irradiated.

The value of A2 was approximately 1.0×10⁻³ N/m.

Example 9

Example 9 will hereinafter be described with reference to FIG. 5. Inproducing the liquid crystal display which is Example 9 of the presentinvention, a glass substrate with a thickness of 0.7 mm and havingpolished surfaces is used as the substrates 101 and 102. The thin filmtransistor 115 is comprised of the source electrode 105, the signalelectrode 106, the scanning electrode 104, and the amorphous silicon116. The scanning electrode 104 was formed by patterning an aluminumfilm. The common electrode wire 120, the signal electrode 106, and thesource electrode 105 were formed by patterning a chromium film. The gateinsulating film 107 and the protecting insulating film 108 were made ofsilicon nitride, and each had a thickness of 0.3 p.m. An acrylic resinwas applied thereon. Heat treatment was performed thereon at 220° C. foran hour to form the transparent and insulating organic protecting film112 with a dielectric constant of approximately 4 and a thickness ofapproximately 1.0 μm. The organic protecting film 112 was used toplanarize irregularities due to the steps of the pixel electrode 105 inthe display area and the steps between adjacent pixels.

Next, photolithography with etch processing was performed to form thecylindrical through-hole reaching the source electrode 105 with adiameter of approximately 10 μm as shown in FIG. 5. The pixel electrode105 connected with the source electrode 105 was formed thereon bypatterning an ITO film. For the common electrode wire 120, a cylindricalthrough-hole was formed with a diameter of approximately 10 μm, and anITO film was patterned thereon to form the common electrode 103. At thispoint, the interval between the pixel electrode 105 and the commonelectrode 103 was set to 7 μm. The electrodes except the scanningelectrode 104 were formed in a zigzag bending electrode wiring pattern.The angle of the bending was set to 10 degrees. The common electrode 103was formed in a lattice form to lie above and cover the video signalwire 106, the scanning signal wire 104, and the thin film transistor 115to surround the pixel such that it also serves as a light shield layer.

As a result, similarly to Example 4 except the two through-holes formedin the unit pixel, the pixel electrode 105 was disposed between thethree common electrodes 103 to form the active matrix substrate with thenumber of pixels of 1024×3×768 constituted by 1024×3 (for R, G, and B)signal electrodes 106 and 768 scanning electrodes 104.

As described above, in the same manner as in Example 4 except the pixelstructure and the alignment control film used, the liquid crystaldisplay of Example 9 was produced as shown in FIG. 5. For the alignmentcontrol film used in Example 9, polyamic acid varnish formed bycombining 2,6-diamino, 9,10-dimethylanthracene shown in a formula [53]and 4,4-diaminobenzophenone shown in a formula [54] as diamine with amole fraction of 2:1 and 1,2,3,4-cyclobutanetetracarboxylic aciddianhydride shown in a formula [55] and1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shown in a formula[56] with a mole fraction of 1:2 was used to form the alignment controlfilm with a thickness of approximately 20 nm.

Then, when the display quality of the liquid crystal display which isExample 9 of the present invention was evaluated, high quality displayequivalent to the liquid crystal display of Example 1 was observed and awide viewing angle at halftone display was observed. Next, in the samemanner as in Example 1 of the present invention, the image persistenceand after-image relaxation time of the liquid crystal display ofComparative Example 1 were quantitatively evaluated, and the after-imagerelaxation time was found equal to shorter than one minute. Also, in thevisual image quality after-image test, an excellent displaycharacteristic was found without any uneven image due to imagepersistence or after-image.

As shown in FIG. 5, when the pixel electrode directly connected to theTFT is formed closest to the surface of the substrate and the thinalignment control film is formed thereon, normal rubbing alignmentprocessing may cause charging due to friction, and in some cases, theTFT device may be damaged through the pixel electrode near the surface.In such a case, the rubbing-less photo-alignment processing as Example 9is greatly effective.

The value of A2 was approximately 8.1×10⁻⁴ N/m.

Example 10

FIG. 6 is a schematic section view showing almost one pixel forexplaining Example 10 of the liquid crystal display according to thepresent invention. In producing the liquid crystal display of Example10, a glass substrate with a thickness of 0.7 mm and having polishedsurfaces is used as the glass substrates 101 and 102. The thin filmtransistor 115 is comprised of the pixel electrode 105, the signalelectrode 106, the scanning electrode 104, and the amorphous silicon116. All of the scanning electrode 104, the common electrode wire 120,the signal electrode 106, the pixel electrode 105, and the commonelectrode 103 were formed by patterning a chromium film. The intervalbetween the pixel electrode 105 and the common electrode 103 was set to7 μm. The gate insulating film 107 and the protecting insulating film108 were formed of silicon nitride and each had a thickness of 0.3 μm.As the alignment control film, polyamic acid varnish consisting of9,10-diaminoanthracene shown in a formula [57] as a diamine compound and1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shown in a formula[58] as acid dianhydride was formed on the substrate surface throughprinting and fired at 230° C. for 30 minutes for imidation to depositthe film with a thickness of approximately 20 nm.

Thereafter, while the surface was irradiated in vacuum with electronbeams at 5 eV and approximately 0.5 μC/cm², light from a high pressuremercury lamp was changed through an interference filter and a quartzpile polarizer into polarized UV with a wavelength range from 220 nm to380 nm and was irradiated with an irradiation energy of approximately 3J/cm² to perform photo-alignment processing. As a result, the activematrix substrate was formed with the number of pixels of 1024×3×768constituted by 1024×3 (for R, G, and B) signal electrodes 106 and 768scanning electrodes 104. As described above, similarly to Example 1except the pixel structure, the liquid crystal display of Example 10 asshown in FIG. 6 was produced.

When the display quality of the liquid crystal display of Example 10 wasevaluated, high quality display equivalent to the liquid crystal displayof Example 1 was observed and a wide viewing angle at halftone displaywas observed. Next, in the same manner as in Example 1 of the presentinvention, the image persistence and after-image relaxation time of theliquid crystal display of Example 10 were quantitatively evaluated, andthe after-image relaxation time was found equal to shorter than twominutes. Also, in the visual image quality after-image test, defectivedisplay due to image persistence or after-image was not observed. Inaddition, when polyamic acid varnish combined by introducing1,5-diethyl-9,10-diamineoanthracene shown in a formula [59] which is aderivative of the diamine compound used in Example 10 with a molefraction of 50% was used, an equivalent excellent display characteristicwas provided with an irradiation energy of polarized UV of approximately2 J/cm².

The value of A2 was approximately 6.0×10⁻⁴ N/m.

Example 11

In the same manner as in Example 10 except the composition of thealignment control film used, the formation of the alignment controlfilm, and the alignment processing process, polyamic acid varnishconsisting of 2,7-diaminophenanthrene shown in a formula [60] as adiamine compound of the alignment control film of Example 11 and a1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shown in a formula[61] as acid dianhydride was formed through printing on the substratesurface. Labeling was performed through heat processing at 90° C. fortwo minutes to deposit the film with a thickness of approximately 35 nm.Then, while the surface was irradiated with far infrared rays and thefilm surface was held at approximately 230° C., light from a highpressure mercury lamp was changed through an interference filter and aquartz pile polarizer into polarized UV with a wavelength range from 220nm to 380 nm and was irradiated with an irradiation energy ofapproximately 3 J/cm² to perform photo-alignment processing. Thethickness of the processed alignment control film was approximately 25nm.

Then, the liquid crystal display of Example 11 as shown in FIG. 6 wasproduced similarly to Example 10 and a nematic liquid crystalcomposition A was filled. Annealing was then performed at 100° C. for 10minutes. Thus, favorable liquid crystal alignment was provided in adirection substantially perpendicular to the abovementioned polarizationdirection of the irradiation. In this manner, the liquid crystal displayhaving the liquid crystal layer with a thickness d of 4.0 μm wasprovided. A cell of homogeneous alignment was produced by using analignment control film and a liquid crystal composition equivalent tothose used in this panel. The measurement of the pretilt angle of theliquid crystal with the crystal rotation technique showed approximately0.1 degrees.

Then, when the display quality of the liquid crystal display of Example11 was evaluated in the same manner as in Example 1, light leakage dueto defective alignment near the electrode steps seen typically in therubbing alignment processing was not found and high quality display wasobserved with a contrast ratio of more than 600:1 over the entirescreen, which is equivalent to that of the liquid crystal display ofExample 1, and a wide viewing at halftone display was also observed. Inthe same manner as in Example 1, the image persistence and after-imagerelaxation time of the liquid crystal display of Example 11 werequantitatively evaluated, and the after-image relaxation time was foundequal to shorter than one minute. Also, in the visual image qualityafter-image test, an excellent display characteristic was found withoutany uneven display due to image persistence or after-image.

The value of A2 was approximately 7.2×10⁻⁴ N/m.

Example 12

In the same manner as in Example 9 except the composition of thealignment control film used, the formation of the alignment controlfilm, and the alignment processing process, polyamic acid varnishconsisting of 9,10-diaminoanthracene shown in a formula [62] as adiamine compound of the alignment control film of Example 12 and a1,2,3,4-cyclobutanetetracarboxylic acid dianhydride shown in a formula[63] as acid dianhydride was formed through printing on the substratesurface. Labeling was performed through heat processing at 90° C. for 2minutes to deposit the film with a thickness of approximately 30 nm.Then, while the surface was irradiated with far infrared rays and thefilm surface was held at approximately 240° C., light from a highpressure mercury lamp was changed through an interference filter and aquartz pile polarizer into polarized UV with a wavelength range from 220nm to 280 nm and irradiated with an irradiation energy of approximately3 J/cm² to perform imidation burning processing and photo-alignmentprocessing. The thickness of the processed alignment control film wasapproximately 26 nm.

Then, the liquid crystal display of Example 12 as shown in FIG. 5 wasproduced in the same manner as in Example 9 and a nematic liquid crystalcomposition A was filled. Annealing was performed at 100° C. for 10minutes. Thus, favorable liquid crystal alignment was provided in adirection substantially in parallel with the abovementioned polarizationdirection of the irradiation. In this manner, the liquid crystal displayhaving the liquid crystal layer with a thickness d of 4.0 μm wasprovided. A cell of homogeneous alignment was produced by using analignment control film and a liquid crystal composition equivalent tothose used in this panel. The measurement of the pretilt angle of theliquid crystal with the crystal rotation technique showed approximately0.1 degrees.

Then, when the display quality of the liquid crystal display which isExample 7 of the present invention was evaluated in the same manner asin Example 1, light leakage due to defective alignment near theelectrode steps seen typically in the rubbing alignment processing wasnot found and high quality display was observed with a contrast ratio ofmore than 600:1 over the entire screen, which is equivalent to that ofthe liquid crystal display of Example 1, and a wide viewing angle athalftone display was observed. In the same manner as in Example 1, whenthe image persistence and after-image relaxation time of the liquidcrystal display of Example 12 were quantitatively evaluated, theafter-image relaxation time was equal to shorter than one minute in arange of operating temperatures from 0 to 50° C. Also, in the visualimage quality after-image test, an excellent display characteristic wasfound without any uneven display due to image persistence orafter-image.

Example 13

Next, Example 13 will be described with reference to FIGS. 7 and 8 asthe specific structure of the liquid crystal display which is Embodiment4 of the present invention.

In producing the liquid crystal display which is Example 13 of thepresent invention, a glass substrate with a thickness of 0.7 mm andhaving polished surfaces is used as the substrate 101. On the substrate101, the insulating film 107 for preventing the short circuit of theelectrodes 103, 105, 106, and 104, the thin film transistor 115, and theprotecting insulating film 108 for protecting the thin film transistor115 and the electrodes 105 and 106 are formed to provide the TFTsubstrate.

FIG. 8 shows the structure of the thin film transistor 115 and theelectrodes 103, 105, and 106.

The thin film transistor 115 is comprised of the pixel electrode 105,the signal electrode 106, the scanning electrode 104, and the amorphoussilicon 116. The scanning electrode 104 was formed by patterning analuminum film. The signal electrode 106 was formed by patterning achromium film. The common electrode 103 and the pixel electrode 105 weremade by patterning an ITO film.

The insulating film 107 and the protecting insulating film 108 were madeof silicon nitride, and had thicknesses of 0.2 μm and 0.3 μm,respectively. A capacitive element is formed as the structure in whichthe pixel electrode 105 and the common electrode 103 sandwich theinsulating films 107, 108.

The pixel electrode 105 is arranged in an overlapping form in the layerhigher than the common electrode 103 in a uniform shape. The number ofpixels is 1024×3×768 constituted by 1024×3 (for R, G, and B) signalelectrodes 106 and 768 scanning electrodes 104.

On the substrate 102, the color filter 111 with the black matrix 113 ofthe structure similar to the liquid crystal display which is Example 1of the present invention was formed to provide the opposite color filtersubstrate.

Next, as the alignment control film, polyamic acid varnish consisting of4,4′ diaminodiphenylamine and1,3-dichloro-1,2,3,4-cyclobutanetetracarboxylic acid dianhydride wasprepared as 5% by weight for the concentration of the resin, 40% byweight of NMP, 40% by weight of γ-butyrolactone, 15% by weight of butylcellosolve, and formed through printing on the abovementioned activematrix substrate. Heat treatment was performed at 220° C. for 30 minutesfor imidation to form the fine polyimide alignment control film 109 ofapproximately 70 nm.

In the same manner, similar polyamic acid varnish was also formedthrough printing on the surface of the other glass substrate 102 onwhich the ITO was deposited. Heat treatment was performed at 220° C. for30 minutes to form the polyimide alignment control film 109 made of thefine polyimide film of approximately 70 nm. To provide the liquidcrystal alignment ability for the surface, the polyimide alignmentcontrol film 109 was irradiated with polarized UV (ultraviolet rays)light while the surface was irradiated with far infrared rays. A highpressure mercury lamp was used as a light source. The UV light in arange from 240 nm to 380 nm was taken through an interference filter andchanged by a pile polarizer of stacked quartz substrates into linearlypolarized light with a polarization ratio of approximately 10:1 beforeirradiation with an irradiation energy of approximately 5 J/cm². Thetemperature of the alignment control film at this point wasapproximately 120° C.

As a result, the alignment direction of the liquid crystal molecules onthe surface of the alignment control film was found perpendicular to thepolarization direction of the irradiated polarized UV light.

The alignment directions of the alignment control films 109 on the TFTsubstrate and the color filter substrate were substantially in parallelwith each other and formed an angle of 15 degrees with respect to thedirection of the applied electric field 117. Polymer beads with anaverage diameter of 4 μm were dispersed as a spacer between thesubstrates, and the liquid crystal 110 was sandwiched between the TFTsubstrate and the color filter substrate. The liquid crystal compositionA identical to that of Example 1 was used as the liquid crystal 110.

The two polarizing plates 114 for sandwiching the TFT substrate and thecolor filter substrate were arranged as crossed nicols. The normallyclose characteristic was employed in which a dark state was shown at lowvoltage and a bright state was shown at high voltage.

Since the structure of the system for driving the liquid crystal displaywhich is Example 13 of the present invention is similar to that ofExample 1, the details of the structure are omitted.

Then, when the display quality of the liquid crystal display which isExample 13 of the present invention was estimated, the aperture ratiowas higher than that of the liquid crystal display of Example 1, highquality display with a contrast ratio of 650:1 was observed, and a wideviewing angle at halftone display was also observed. In addition, in thesame manner as in Example 1 of the present invention, the imagepersistence and after-image relaxation time of the liquid crystaldisplay were quantitatively evaluated, the after-image relaxation timewas approximately one minute in a range of operating temperatures from 0to 50° C. Also, in the visual image quality after-image test, anexcellent display characteristic equivalent to that of Example 1 wasfound without any uneven display due to image persistence orafter-image.

In addition, when the anchoring energy A2 in the azimuth direction atthe interface between the liquid crystal and the alignment film wasevaluated in the same manner as in Example 1, the value equal to orhigher than approximately 1.0×10⁻³ N/m was achieved.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, in the liquidcrystal display of the IPS scheme, it is possible to prevent the liquidcrystal display capable of solving the inherent problem of the smallproduction margin in the alignment processing, reducing the occurrenceof defective display due to variations in the initial alignmentdirection, realizing the stable liquid crystal alignment, providingexcellent mass productivity, and having high image quality with a highercontrast ratio.

1. An photo alignment film material for an IPS type liquid crystal display device, adapted to be an alignment control film of a liquid crystal display to drive a liquid crystal with an electric field arising between a pair of electrodes formed on a substrate, comprising a polyamic acid or polyimide that includes cyclobutanetetracarboxylic acid dianhydride and/or its derivative and aromatic diamine; and wherein the cyclobutanetetracarboxylic acid dianhydride and/or its derivative is a compound represented by a formula [1]:

where R₁, R₂, R₃ and R₄ of the compound of the formula [1] each independently represent a hydrogen atom, a fluorine atom, an alkyl group or alkoxyl group with a carbon number of 1 to 6, with the proviso that at least one of R₁, R₂, R₃ and R₄ of the compound of formula [1] is not hydrogen, wherein the aromatic diamine compound contains at least one of compounds selected from a group of compounds consisting of ones represented by formulas [2] to [16]:

where R₁, R₂, R₃ and R₄ of the compounds represented by formulas [2] to [16] each independently represent a hydrogen atom, a fluorine atom, an alkyl group or alkoxyl group with a carbon number of 1 to 6, or vinyl group {—(CH₂)_(m)—CH═CH₂, m=0, 1, 2} or a group represented by {—(CH₂)_(n)—C≡CH, n=0, 1, 2}, and in the formula [5], X represents a bond group —S—, —CO—, or —NH—.
 2. The photo alignment film material for an IPS type liquid crystal display device according to claim 1, wherein said alignment control film is polyamic acid or polyimide containing at least 50% of a repeated structure of polyamic acid or polyimide comprising said cyclobutanetetracarboxylic acid dianhydride and/or its derivative and said aromatic diamine.
 3. The photo alignment film material for an IPS type liquid crystal display device according to claim 1, having a glass transition temperature equal to or higher than 250° C.
 4. The photo alignment film material for an IPS type liquid crystal display device according to claim 1, wherein said polyamic acid or polyimide is a photoreactive polyamic acid or polyimide.
 5. The photo alignment film material for an IPS type liquid crystal display device according to claim 1, said film having been subjected to alignment processing by irradiating the film with polarized light having a wavelength in a range of 200 to 400 nm.
 6. The alignment control film according to claim 5, wherein said alignment processing further comprises applying at least one of heating, irradiation of infrared rays, irradiation of far infrared rays, irradiation of electron beams and irradiation of radioactive rays when irradiating the film with the polarized light.
 7. The photo alignment film material for an IPS type liquid crystal display device according to claim 1, adapted to be an alignment control film of a liquid crystal display having an electric field parallel to said substrate.
 8. The photo alignment film material for an IPS type liquid crystal display device according to claim 1, wherein the alignment control film has a thickness of from 1 nm to 100 nm.
 9. A varnish comprising the photo alignment film material according to claim 1 mixed with an organic solvent.
 10. The photo alignment film material for an IPS type liquid crystal display device according to claim 8, wherein said thickness is 1 to 30 nm. 